Acridinium esters, acridine compounds, are useful as chemiluminescent labeling substances for their possession of high efficiencies of luminescence. Use of such an acridinium ester as a chemiluminescent label in an immunoluminescent analysis for a clinical test is disclosed, for example, in European Patent Publication No. 82636 and U.S. Pat. No. 4,745,181.
Known acridinium esters include those having hydrophilic structures such as those containing sulfonium ions at ends [Japanese Patent Application Laid-Open (Kokai) No. HEI 5-255264], those containing hydrazonium ions in spacer portions [Japanese Patent Application Laid-Open (Kokai) No. HEI 5-255263], and those obtained by substituting a carboxyl group for the methyl group or the like of acridinium [Japanese Patent Application Laid-Open (Kokai) No. HEI 6-228102]. These acridinium esters having hydrophilic structures are practically intended to label amino groups, and are considered to be compounds suitable for labeling amino acids, proteins and the like and hence for use in immunoluminescent analyses for clinical tests.
Incidentally, analytes in immunoluminescent analyses for clinical tests are mostly amino acids and proteins. As these substances contains many amino groups, the above-described known acridinium compounds have a significant advantage in that labeling can be easily performed.
In contrast, they label amino groups contained abundantly in amino acids and proteins. As a result, there are many site to be labeled. This has led to problems in the uniformity and reproducibility of labeling, the problem of insolubilization of analytes such as antibody proteins, and a problem that, when labeling is effected to an antibody, a labeling compound binds to amino groups located at antigen recognition sites and its function as an antibody is reduced or lost. When an analyte is a low-molecular substance, that is, contains only a limited number of amino groups, it may be possible to control a labeling reaction and to readily perform labeling at a constant molar ratio. However, when an analyte is a high-molecular substance such as an antibody protein and the number of amino groups cannot be precisely determined, there is a drawback that conditions for permitting labeling at a constant molar ratio have to be provisionally ascertained through repeated trial and error.
Incidentally, to furnish a compound as a chemiluminescent labeling reagent for practical use, it is essential that the compound assures an easy labeling reaction and does not result in luminescence or decomposition under labeling conditions. Counterparts to be labeled by an acridinium ester vary widely, led by low-molecular compounds such as amino acids and including even high-molecular compounds such as enzymes and antibodies. When amino groups of an analyte are relied upon as described above, an imide group is often introduced into an acridinium ester to make it bind to amino groups as disclosed in the publications referred to above. When a binding reaction to amino groups is performed using this imide group, the reaction proceeds efficiently under alkaline conditions. However, an acridinium compound is an unstable compound so that it results in luminescence or decomposition under alkaline conditions. Efficient performance of labeling while retaining luminescent activity therefore requires mutually contradictory reaction conditions and is not conveniently feasible.
It has accordingly been desired to find out a labeling compound capable of binding to an analyte such as an amino acid or protein under mild conditions in a chemiluminescent labeling method useful in an immunoluminescent analysis or the like and having sufficient binding force while possessing high specificity, and further to provide a method for stably and accurately detecting the analyte by making use of the labeling compound.